A light source device for an endoscope emits illumination light from a light source, and the illumination light is supplied to the endoscope through a universal cord. In the endoscope, the illumination light is introduced, through a light guide, to a distal portion of an insert section, and thereby illuminating inside a human body cavity. When the illumination light is emitted for a long time, optical members and other components of the light source device may be heated up and damaged. In view of this, the light source device is provided with a light quantity controller for controlling the quantity of the illumination light. In the endoscope having an imaging sensor at the distal portion of the insert section, increase in a heating value may cause thermal noise to the imaging sensor, which may lower image quality or damage the material of the distal portion of the insert section. Therefore, such an endoscope especially needs to adjust the quantity or illumination periods of the illumination light. In addition, the light guide provided inside the endoscope is constituted of a bundle of optical fibers. The quantity of the illumination light guided to the distal portion of the insert section increases with increase in the number of the optical fibers. As a result, the heating value is raised and its effects become severe.
Such light source devices provided with the above-described light quantity controller are disclosed in, for example, Japanese Patent Application Laid-open Publications No. 8-15617 and No. 2000-56240. The light source devices disclosed in these publications include a light quantity detection circuit for detecting light quantity based on an imaging signal output from an imaging sensor of an endoscope and a controller for making a light source perform pulse-lighting such that the light source emits the illumination light in synchronization with exposure periods of the imaging sensor.
In addition, a light source device disclosed in Japanese Patent Application Laid-open Publication No. 2000-189383 is constituted of an endoscope having a light guide and an optical guide, and a TV camera connected to a rear end of the endoscope. Illumination light from the light guide illuminates a human body cavity. An image of the human body cavity is transmitted to the TV camera through the optical guide and is converted into an image signal by an imaging sensor inside the TV camera.
In Japanese Patent Application Laid-open Publication No. 2000-189383, the illumination light from a light source is transmitted through the light guide of the endoscope. The light source device is provided with an aperture adjustment mechanism for adjusting the illumination light from the light source and a judgment device for judging whether to control the illumination light based on variation in brightness distribution generated from a video signal output from the imaging sensor of the TV camera. When the judgment device judges as the control of the illumination light is needed, the aperture adjustment mechanism puts an aperture blade into a close position to block the illumination light.
In the light source device of Japanese Patent Application Laid-open Publication No. 2000-189383 provided with the aperture adjustment mechanism, the quantity of the illumination light for illuminating the human body cavity is reduced when the aperture is made small. As a result, the image becomes dark and cannot be observed well on a monitor. In view of this, an electronic endoscope disclosed in Japanese Patent Application Laid-open Publication No. 2007-195850 has a rotary shutter having a rotating light-blocking plate and a controller for controlling the rotary shutter, in addition to the light quantity controller like the aperture adjustment mechanism. For this configuration, illumination periods for illuminating a subject and light-blocking periods for blocking the illumination light are alternately repeated by rotating the rotary shutter. At this time, the rotary shutter is controlled such that the illumination periods are synchronized with the exposure periods of the imaging sensor. Owing to this, sufficient quantity of light for illuminating the subject can be obtained with simple configuration and the heating value can be lowered by controlling the illumination periods for illuminating the subject.
Meanwhile, as the light source for the light source device, a discharge lamp, such as a xenon lamp, featuring high brightness and long life is used instead of a halogen lamp in recent years. At the time of lighting up the xenon lamp, high voltage is applied to a xenon tube to start an electric discharge. After completing the light up, steady voltage which is lower than the voltage applied at the time of discharging is applied to keep illuminating the lamp.
In the light source devices disclosed in Japanese Patent Application Laid-open Publications No. 8-15617 and No. 2000-56240, the quantity of the illumination light is adjusted by performing the pulse-lighting of the light source. However, when the above-described xenon lamp is used as the light source, it is hard to control the illumination periods because the xenon lamp takes time to be lit up once it is turned off. In addition, the xenon lamp may consecutively generate the high-frequency noise each time the lamp is lit up in the pulse-lighting.
The rotary shutter like the one disclosed in Japanese Patent Application Laid-open Publication No. 2007-195850 is advantageous in view of cost and mechanical properties as compared to the pulse-lighting or the aperture adjustment mechanism. However, when the xenon lamp is used for the light source device having the rotary shutter, a driver circuit of the rotary shutter may have malfunction due to high-frequency noise which occurs at the time of starting the electric discharge of the xenon lamp. The malfunction may cause loss of synchronization of a motor of the rotary shutter. When the loss of synchronization occurs, the rotating light-blocking plate may stop at a position corresponding to the light-blocking period and the illumination light may be blocked even in the illumination period, and then the monitor screen may turn black.